[clang] [lld] [llvm] [llvm][clang][lld][COFF] Add support for case-insensitive path lookup (PR #178585)

[Hans Wennborg via cfe-commits]

https://github.com/zmodem commented:

IIUC what I got stuck on when working on this the last time was that enabling this by default (to verify that it worked) broke a bunch of modules related tests, because `CaseInsensitiveFileSystem::Maps`, which acts as a cache, didn't see updates to the file system that didn't go through that vfs.

Maybe that's less of a concern now after https://github.com/llvm/llvm-project/pull/171935, but it would be good to test it.

Also I didn't consider non-ASCII at all. I don't have any strong opinions on the discussion in #161400 but I do note that this keeps coming up (my patch was based on Saleem's https://reviews.llvm.org/D2972 from 2014), so I think it would be useful if we can make it good enough.

https://github.com/llvm/llvm-project/pull/178585